1. Field of the Invention
The present invention relates to a light-receiving element used in, e.g., an optical communication system such as an optical LAN(local area network) or an optical CATV system.
2. Description of the Related Art
Conventionally, a light signal receiving section in an optical communication system of this type generally employs a circuit arrangement, as shown in FIG. 1. When a light signal is incident on a light-receiving element 1, the light-receiving element 1 generates a light output current. The light output current is converted into a voltage signal by a resistor 2, and the voltage signal is supplied to an amplifier 3. The incident light signal is amplified and demodulated by the amplifier 3, and the demodulated signal is output. The output signal from the amplifier 3 is also supplied to an amplifier 4. The output terminal of the amplifier 4 is connected to diodes 5 and 6, a resistor 7, and a capacitor 8. The output signal supplied to the amplifier 4 is output to a terminal 9 as a monitor signal. Most optical communication systems have a function of monitoring the presence/absence of a light input.
FIG. 2A is a plan view of the light-receiving element 1, and FIG. 2B is a longitudinal sectional view of FIG. 2A. The light-receiving element 1 is formed to have a PIN structure. As an uppermost layer of the PIN structure, an SiO.sub.2 (silicon oxide) film 10 is formed. A light signal emitting from an optical fiber is radiated on an active area 11 indicated by hatching in FIG. 2A. A metal wall 12 is formed around the active area 11. Upon radiation of the light signal, the PIN structure generates a light output current. The light output current is output through a lead terminal formed by the metal wall 12.
In the conventional light signal receiving circuit, however, since an electrical signal output from the amplifier 3 is branched to the amplifier 4 and serves as a monitor signal, the light output current generated by the light-receiving element 1 is also consumed by the amplifier 4. Therefore, if incident light power is very weak, a disturbance occurs in a low-level output electrical signal obtained from the amplifier 3. For this reason, information transmission in the optical communication system cannot be accurately performed.